This past weekend, installed a sub and amp into my SC300. I used speaker level inputs taken from the stock sub. When the radio is turned all the way down, or is between switching stations, the sub begins to vibrate heavily, but only when the trunk is closed or close to being closed.

I checked to see if any wires were touching when the trunk was being closed down and everything is clear. What could be the cause of this? It only happens when the volume is silent.